1. Field of the Invention
The present invention relates to a back door for an automobile.
2. Related Background Art
Weight reduction of automotive parts has recently been demanded for the purpose of improving fuel efficiency of automobiles and for reducing environmental loads. There are an increasing number of cases where structural materials of the parts are changed from metal to resin for weight reduction. Resin back door members have also been introduced. To ensure strength, resin back doors are known to be formed by press-molding glass mats (for example, see Japanese Patent Application Laid-Open Publication No. 2001-206062). A method of molding a back door by injection molding using a thermoplastic resin containing long fiber is also disclosed (see Japanese Patent Application Laid-Open Publication No. 2007-30600).
Resin inner panels of automotive back doors are advantageous in that the specific rigidity is higher than that of steel plate products (lighter than steel plate products in terms of flexural rigidity), that a complex shape that is difficult to form with sheet metal in steel plate products can be formed relatively easily, and that they are suitable for small-batch production because their molds are cheap although the product cost is high.
A resin back door is generally fixed to a car body with a hinge mounted at an upper portion thereof and a lock set at the center of a lower portion thereof. Dampers for holding the back door being open and for assisting in opening/closing the door are set at both sides. Force on the damper is directly exerted on the back door and may cause the back door to be deformed because the damper set position and the hinge attachment position are set apart from each other. More specifically, as shown in FIG. 7, an attachment position of a member (a stud ball 17) for connecting a damper 18 and an attachment position of a hinge 1 are provided away from each other in an inner panel 2 which is a resin molded product. Therefore, when the back door is open, the force from the hinge and the damper acts on points apart from each other through the back door itself. As a result, the back door, if not having sufficient rigidity, may be deformed. In order to overcome this problem, a metal reinforcement member may be attached on the entire back door. However, this method may involve mass increase and cost increase. As another method, a material having rigidity as high as a steel plate may be used as the material of the inner panel. However, in this method, the flexibility in shape may be impaired.